cp's OEIS Frontend

A036325 Composite numbers whose prime factors have no digits other than 8 and 9.

%I A036325 #31 May 22 2022 05:49:33
%S A036325 7921,704969,800911,8001011,8009021,8802011,8810911,8899021,62742241,
%T A036325 71281079,79120021,80001121,80982001,88109911,88910021,712089979,
%U A036325 712802869,783378979,784171079,791120021,791200121,792012869,800020021,800109911,800901121,800991011,809001101,809811011,880111121
%N A036325 Composite numbers whose prime factors have no digits other than 8 and 9.
%C A036325 All terms are a product of at least two terms of A020472. - _David A. Corneth_, Apr 30 2018
%H A036325 David A. Corneth, <a href="/A036325/b036325.txt">Table of n, a(n) for n = 1..10000</a> (first 5375 terms from Robert Israel)
%H A036325 <a href="/index/Pri#prime_factors">Index entries for sequences related to prime factors</a>.
%F A036325 Sum_{n>=1} 1/a(n) = Product_{p in A020472} (p/(p - 1)) - Sum_{p in A020472} 1/p - 1 = 0.0001296249159... . - _Amiram Eldar_, May 22 2022
%e A036325 7921 is in the sequence because it's composite and its only prime factor is 89, only having digits 8 or 9. - _David A. Corneth_, Apr 30 2018
%p A036325 N:= 9: # to get all terms of <= N digits
%p A036325 R:= 10^N: G:= {9}: S:= {1}:
%p A036325 for n from 1 to N-1 do
%p A036325   G:= map(t -> (t+8*10^n,t+9*10^n), G);
%p A036325   newprimes:= select(isprime, G);
%p A036325   for p in newprimes do
%p A036325     S:= map(s -> seq(s*p^i,i=0..floor(log[p](R/s))), S)
%p A036325   od
%p A036325 od:
%p A036325 sort(convert(remove(isprime, S minus {1}),list)); # _Robert Israel_, Apr 30 2018
%Y A036325 Cf. A020472, A036302-A036324.
%K A036325 nonn,easy,base
%O A036325 1,1
%A A036325 _Patrick De Geest_, Dec 15 1998
%E A036325 More terms from _Robert Israel_, Apr 29 2018